1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device and, more particularly, to a flash memory device and a program method of the flash memory device in which a program voltage is applied to a selected word line while a local voltage is applied another word line.
2. Description of the Related Art
A flash memory device known as a flash Electrically Erasable and Programmable Read Only Memory (EEPROM) has a memory cell array that may include memory cells having floating gate transistors. The memory cell array may consist of memory blocks, each of which includes bit lines arranged in parallel and multiple strings (e.g., referred to as “NAND strings”) respectively corresponding to the bit lines. Each of the strings has a string select transistor, a ground select transistor and multiple floating gate transistors connected in series between the select transistors. The multiple floating gate transistors in each string are formed on the same substrate, and are configured to share a source-drain terminal with adjacent floating gate transistors. Multiple word lines are arranged so that they intersect with the strings. Each word line is connected to control gates of the multiple floating gate transistors in each of corresponding rows.
In order to program memory cells having floating gate transistors, the memory cells are first erased to have a given threshold voltage (e.g., −3V). A program operation may then be carried out with respect to selected memory cells by applying a high voltage (e.g., 20V) to a word line connected to the selected memory cells. During the program operation, threshold voltages of the selected memory cells may be shifted to a higher threshold voltage, while threshold voltages of unselected memory cells may remain unchanged.
However, because control gates of the floating gate transistors in the same row are connected to a corresponding word line, a program voltage may be applied to both selected and unselected memory cells when the program voltage is applied to a selected word line. Accordingly, the unselected memory cells positioned along the same word line, especially memory cells adjacent to the selected word line, may be unintentionally programmed. Unintended programming of unselected memory cells may be called “program disturbance.”
Techniques for preventing program disturbance may include, for example, program-inhibit methods using a self-boosting scheme and a local self-boosting scheme. Exemplary program-inhibit methods using a self-boosting scheme are disclosed in U.S. Pat. No. 5,677,873, entitled “METHODS OF PROGRAMMING FLASH EEPROM INTEGRATED CIRCUIT MEMORY DEVICES TO PREVENT INADVERTENT PROGRAMMING OF NONDESIGNATED NAND MEMORY CELLS THEREIN,” and U.S. Pat. No. 5,991,202, entitled “METHOD FOR REDUCING PROGRAM DISTURB DURING SELF-BOOSTING IN A NAND FLASH MEMORY.” Exemplary program-inhibit methods using a local self-boosting scheme are disclosed in U.S. Pat. No. 5,715,194, entitled “BIAS SCHEME OF PROGRAM INHIBIT FOR RANDOM PROGRAMMING IN A NAND FLASH MEMORY,” and U.S. Pat. No. 6,061,270, entitled “METHOD FOR PROGRAMMING A NON-VOLATILE MEMORY DEVICE WITH PROGRAM DISTURB CONTROL.” The program-inhibit method using the local self-boosting scheme may be suitable for increasing a channel boosting voltage of a program-inhibited memory cell transistor, as compared to the program-inhibit method using the self-boosting scheme. Therefore, the program-inhibit method using the local self-boosting scheme has been widely used to program multi-level cells.
However, despite utilization of the above-referenced program-inhibit methods, program disturbance remains a factor as flash memory devices become more highly integrated. This is because the intervals between memory cells and between signal lines are reduced when the degree of integration increases. In other words, the probability increases that coupling may arise. As a result, the program disturbance may affect not only selected memory cells and unselected memory cells in the same word line, but also memory cells in the same string as selected memory cells.